Online business

How can one build a successful online business? And build trust with customers?
Level 9
To answer your question more directly I'm sure things like the following are helpful in understanding where you're starting at;

Will there be shipping? Have you adjusted for the costs?

Will you be selling wholesale brands or creating your own?

Niche items only?

At the end of the day it's all about the amount of sales and happy customers (in turn good reputation)

Over time, this creates trust via word of mouth.

I'm sure users with much more experience will chime in.

At the bare minimum, there are a couple things you can do from your end currently in regards to advertising specifically.

You can create a blog / website with the items you are selling and link them in yout twitter / instagram etc.

eBay is fine if you're doing wholesale.

Create banners for smf / php type forums and link your offerup profile to it.

Make sure they're simple yet eye catching designs to get your brand or name out there.

The key is to create banners that are not instrusive / obnoxious.

You can offer referrals (discounts) to any customers who rate you well and those who don't polietly ask why they were not satisfied.

Encourage your buyers to give you critisim on how you can improve.